1. Field of the Invention
The present invention relates to a motor control device suitably used for, for example, controlling a stepping motor for applying a driving force to a scanning system for scanning a document in a copying machine.
2. Description of the Related Art
A copying machine conforming to an electrophotographic process so adapted as to illuminate and scan a document and introduce light reflected from the document in the process of the scanning into a photoreceptor to form an electrostatic latent image corresponding to the document on the surface of the photoreceptor has been conventionally used. The electrostatic latent image on the surface of the photoreceptor is developed into a toner image. This toner image is transferred and fixed to copy paper, thereby to achieve copying of the document.
The illumination and scanning of the document is achieved by moving a moving frame supporting a light source and a reflecting mirror from one end to the other end of the document. A driving force from a motor is exerted on the moving frame through a suitable clutch or the like. A stepping motor capable of accurately controlling the rotation is generally used as this motor.
The moving speed of the moving frame is not constant. Specifically, the moving speed of the moving frame differs depending on, for example, the time of copying at equal magnification when the document is copied without changing its original size, the time of enlarged copying when the document is copied in an enlarged manner, the time of reduced copying when the document is copied in a reduced manner, and the time of a return operation when the moving frame which reached the terminal position for movement is returned to the home position which is the initial position. For example, when a photoreceptor in the shape of a right circular cylinder is used, the photoreceptor is driven to rotate at constant speed around its axis, while the moving speed of the moving frame is changed, thereby to make it possible to achieve enlarged copying or reduced copying. Therefore, at the time of enlarged copying, the moving frame is moved at lower speed than that at the time of copying at equal magnification depending on the magnification for enlargement. On the other hand, at the time of reduced copying, the moving frame is moved at higher speed than that at the time of copying at equal magnification depending on the magnification for reduction. Furthermore, at the time of the return operation, the moving frame is moved at high speed so as to prepare for the subsequent copying operation.
The change in the moving speed of the moving frame is achieved by changing the rotation speed of the stepping motor. Specifically, the stepping motor is controlled by a control circuit including a microcomputer or the like. The control circuit changes the pulse period for controlling the stepping motor depending on operations such as the copying at equal magnification, the enlarged copying, the reduced copying and the return operation. This pulse period is changed depending on the magnification at the time of enlargement or reduction. The stepping motor is stopped in a waiting time period immediately before any one of the operations is started.
When the moving frame is driven by the stepping motor, however, torque produced by the stepping motor becomes rather deficient when the stepping motor is rotated at high speed, resulting in the possibility that the moving frame cannot be sufficiently driven. In addition, when the stepping motor is rotated at low speed or is stopped, current supplied to the stepping motor becomes excessive, resulting in the possibility that the stepping motor abnormally generates heat or is damaged.
In order to avoid such programs, chopper constant current control is carried out so that current to be supplied to the stepping motor is held constant irrespective of the rotation speed of the stepping motor. However, experience has shown that a sufficient improvement is not achieved merely by keeping the current to be supplied to the stepping motor constant, so that the torque, the supplied current or the like becomes excessive or deficient.
Conventionally, a chopper constant current control level for determining the current to be supplied to the stepping motor is variably set depending on the rotation speed of the stepping motor at the operation times such as the time of copying at equal magnification, the time of enlarged copying, the time of reduced copying, the return time and the waiting time.
More specifically, the chopper constant current control level at each of the operation times is written in a RAM (Random Access Memory) which is backed up by a battery cell. A control circuit for controlling the stepping motor reads out the chopper constant current control level from the RAM at the operation time, converts the data into an analog signal and supplies the analog signal to a chopper constant current control circuit. Consequently, the chopper constant current control level conforming to the rotation speed of the stepping motor is set, so that necessary torque is obtained at the operation time, and the supplied current cannot be excessive.
The chopper constant current control level is written in the RAM in order to make the change of the level easy so that adjustment and maintenance can be easily performed in each copying machine.
However, the rotation speed of the stepping motor must be varied depending on the magnification at the time of enlarged copying or the time of reduced copying. Even if a constant chopper constant current control level is set with respect to magnification in the whole range for the enlarged copying or the reduced copying, therefore, it is impossible to sufficiently control the stepping motor in the whole range of magnification. For example, the torque becomes excessive or deficient. In addition, even when necessary torque can be obtained over the whole range of magnification, there are possibilities that the stepping motor abnormally generates heat and the stepping motor produces noise in a wide range of magnification.
In order to solve the problems, it is considered that the chopper constant current control level is finely set for each magnification and is stored in the RAM. In this case, however, there occur new problems that a lot of data must be stored in the RAM, and work for making data to be stored becomes very complicated.
Furthermore, at the time of acceleration when the speed of the moving frame is raised or the time of deceleration when the speed of the moving frame is lowered, the rotation speed of the stepping motor is gently changed so as to prevent the step-out of the stepping motor. If a constant chopper constant current control level is so set that necessary torque can be obtained over the whole time period during which the acceleration or the deceleration is achieved, torque is oversupplied to the moving frame in the respective time periods such as a time period immediately after the start of the acceleration and a time period immediately before the termination of the deceleration. Therefore, the speed of the moving frame cannot be gently changed, so that noise due to the vibration of the moving frame is produced.
Additionally, data in the RAM storing the chopper constant current control level is liable to be changed due to electrical noise or the like. In this case, an abnormal chopper constant current control level is set, so that a driving circuit for driving the stepping motor is liable to be destroyed due to overcurrent at the stop time or the time of rotation at low speed.